A composite structure that can sustain a self-propagating reaction is commonly referred to as a metastable intermolecular composite (MIC) material. MICs generally comprise two compositionally different solid materials in intimate contact. These two materials are selected such that upon initiation they are capable of a chemical reaction with one another to form a different material or materials (products), and release heat. An example of a MIC comprising a pair of materials is copper oxide (CuOx; e.g. CuO) and Al, which upon reaction form product materials Cu and Al2O3. The heat released from such a reaction warms the adjacent unreacted composite structure and promotes the rapid reaction of the adjacent regions. Thus, once initiated in one region of the composite structure, the reaction may be sustained and propagate throughout the composite structure. This is often called a “self-propagating” reaction. A MIC is an example of what is referred to as an “energetic” material.
Energetic materials based upon organic (primarily C, H, N, and O) chemistries are used as propellants and explosives by the U.S. military in a large range of weapon systems. Inorganic chemistries, such as used in conventional MIC's, offer similar energy per unit weight of reactants, but can also offer a significant advantage of higher energy per unit volume of reactants (energy density).
Energy density is one of two major performance considerations for applications of energetic materials, the other being the material's reaction velocity, which is also known as the burn rate. Other important considerations for energetic materials include storage lifetime and sensitivity to unwanted (e.g. inadvertent) initiation of the reaction. The product of the energy density and burn rate provides the volumetric reactive power, which is also considered as a performance metric.
The maximum energy density that may be obtained from combustion of MIC's generally depend strongly on the physical form of the composite material. MIC materials prepared from particulates can have densities much less than the theoretical maximum density (TMD) of the materials in their bulk forms. Loose powders typically have densities that are only 5% to 10% of the TMD, and thus negate the energy density advantage of the inorganic energetic materials. Compacted MIC powders generally achieve densities of 60% to 80% of their TMD, and can partially recover the energy density benefits. Layered nano-composite MIC materials are typically fully dense and generally preserve the energy density advantage of the inorganic energetic material.
The burn rate (or reaction velocity) is the second major performance consideration for military and some other applications of energetic materials, and it is significantly enhanced by the use of nanoscale physical forms for the inorganic reactant materials. However, maximum burn rates of conventional organic energetic materials are generally much higher (up to 9,000 m/s) than those of inorganic energetic materials (typically less than 1,000 m/s).
For MIC's using particulate materials, the energy density and burn rate are often inversely related. While burn rates as high as 1,000 m/s have been reported for loose powders (typical densities 5% to 10% of the TMD), burn rates for consolidated powders tend to be significantly lower. The qualitative difference between these two cases can be attributed to the forward convection of hot gases in low density powder assemblies, which is restricted or essentially eliminated in higher density materials.
Physical vapor deposition techniques (such as sputter deposition) have generally been used to manufacture energetic materials, for example by the deposition of alternating layers of Al and CuO thin films within a vacuum chamber. The extent or quality of the vacuum present in such chambers is never perfect, and residual traces of certain contaminant gases are generally always present. The most significant contaminant gas is generally water vapor. Water vapor is known to adsorb readily on surfaces within vacuum chambers and to react with oxidizable metals (such as Al) to form metal oxides (such as Al2O3).
For MIC's that include at least one highly oxidizable material (e.g. Al) and a second material that is a metal oxide formed using physical vapor deposition techniques such as sputtering, there thus exists a thin interfacial region (e.g. interfacial layer) between the two reactant materials that is already reacted highly oxidizable material (e.g. oxidized), prior to any intentional initiation of a self-propagating reaction. Published work presumes or explicitly states that the interfacial reacted zone is always present, generally having a thickness of at least 2-5 nms, such as for an Al/CuO MIC. Moreover, those having ordinary skill in the art generally recognize that such interfacial layers are required to reduce provide stability to the MIC with respect to unintentional initiations.